The present invention relates to an apparatus for detecting a high impedance fault on distribution lines, and more particularly to an apparatus for distinguishing between a zero-sequence current caused by load imbalance and a high impedance fault current caused by incompletely fallen conductors in distribution lines in a multi-ground distribution system or a directly grounded distribution system and for supplying a control signal so as to cut off the flow of load current.
Generally, there are two types of power supply systems for providing currents to a load system through distribution lines. One is a three-phase, four-wire distribution system, the other is a three-phase, three-wire directly grounded system. A protective relay in these systems has in the past used an overcurrent relay for detecting the occurrence of a fault caused by a short circuit in the distribution lines, and an overcurrent ground relay for detecting the occurrence of a fault caused by fallen or broken conductors in distribution lines. Since these relays can block the supply of power which is normally provided to power lines or power equipment, such relays have been commonly used only to prevent an electric shock and to prevent damage to power equipment.
Another type of relay is provided for cutting off the power supply upon detection of a fault where the fault current caused by fallen conductors is compared with a threshold current value that is one-and-a-half times or two times the load current. A fault is detected only when the fault current is greater than the threshold current value.
An overcurrent ground relay is sometimes provided for detecting a zero-sequence current caused by a load imbalance which resembles the type of fault current which characteristically results from fallen conductors, thus detecting a fault on distribution lines.
In a directly-grounded distribution system utilizing such a relay system, fluctuations in load power often occur under normal operation due to an imbalance in the currents in respective phases so that the remaining current appears in a zero-sequence line or a neutral phase line. An amount of the remaining current IN can be obtained from the following relation. EQU IN=Ia+Ib+Ic=310 (1)
whereas Ia, Ib, Ic and IN are A-phase, B-phase, C-phase and neutral-phase current signals, respectively. Accordingly, when an amount of a normal zero-sequence current IN is greater than the threshold value of an overcurrent ground relay for detecting fallen conductors in distribution lines, the relay system malfunctions. Therefore, the relay system has to operate far below the threshold current value so as to prevent malfunction of the system.
The detection of faults in distribution lines is commonly done by devices which measure overcurrents caused by faults. Such devices include a reclosing system including overcurrent relays, overcurrent ground relays, reclosures, or fuses. While these devices interrupt fault currents, they must not trip when normal emergency load currents, such as transient overcurrents, which can be caused by inrush events or load pickup upsurges. Because of this, the threshold of the operation must be set at a relatively high current value to avoid tripping during normal operations. Practically, overcurrent relays in each phase are usually set to operate at 125-200% of maximum load. Therefore, these overcurrent ground relays allow some large neutral currents due to this unbalance. Such conventional art is disclosed in "Detection of high impedance faults on multi-grounded primary distribution system" J. Carr, IEEE Transactions of Power Apparatus and Systems, Vol. PAS-100, pp. 2008-2016, April, 1981.
Although conventional overcurrent devices do detect many faults in the distribution line, these devices still do not detect a large number of faults with very low fault currents (in this case the magnitude of fault currents is nearly 0 to 100A). These faults frequently occur when a fallen distribution conductor comes into contact with a high impedance surface such as an asphalt road, macadam, gravel, sand, or a tree. The fault currents of these high impedance faults are below the threshold of the operation of the fault clearing device. Moreover, since the occurrences of these high impedance faults are increasing due to the frequent use of electrically insulated wire, a better ground relay system for readily detecting the fault is needed.